For signal transmission in optical communication lines, standard single-mode optical fibers having a single light-guiding core are used. Modern telecommunications optical fibers are capable of long-range transmission of huge amounts of data. However, constantly emerging new applications (e.g. HDTV) dramatically (almost exponentially) increase the requirements to the amount of data to be transmitted. To maintain a high data transfer rate all the way to the end user and reduce power consumption, it is necessary to minimize converting the optical signals to electrical signals and back on the path to the end user. This means that each end user should be ideally supplied with a signal propagating over an individual optical fiber. However underground pipes containing optical fibers, especially in large cities, can hold only a limited number of fibers (diameter of pipes between wells), which prevents increasing the number of optical fibers laid in them.
To increase the number of end users supplied with individual channels without renovation/reconstruction of all underground utilities it has been suggested to use several light-guiding cores in a single optical fiber. One such fiber replaces the number of standard optical fibers corresponding to the number of cores.
The main problem in designing multicore fibers is the optical cross-talk, i.e. the interference of the signals carried in one core with the signals carried in other cores.
A conventional fiber comprises seven 8-μm-diameter cores arranged in a hexagonal array. To reduce cross-talk, it is necessary to decrease the interaction between modes of neighboring cores, to this end the cores are arranged far enough (at a distance over 38 microns) from each other (B. Zhu, et al, Seven-core multicore fiber transmissions for passive optical network, Optics Express, Vol. 18, No. 11, pp. 11117-111122 (2010)). The disadvantage of this approach is the big distance between the cores, which allows placing no more than seven cores inside an optical fiber with the standard diameter of 125 microns.
In another conventional fiber structure (M. Koshiba et al, Heterogeneous multicore fibers: proposal and design principle, IEICE Electronics Express, Vol. 6, No. 2, pp. 98-103, 2009), modes of different fiber cores have different propagation constants in order to reduce crosstalk. For this purpose, the cores are made with different diameters. This enables the distance between the centers of cores to be reduced compared to the optical fiber with identical cores, while maintaining an acceptable level of crosstalk. With the core diameter of about 9 microns the distance between the cores is 35 microns, and with the core diameter of about 5 microns it is 20 microns. The disadvantage of this method is the difficulty of joining the fibers to other fibers, because when joining the fiber to another optical fiber it is necessary not just to join the cores with each other, but also to find cores of each particular type and to align exactly them.
Moreover, it was found that bends of multicore optical fibers, even with relatively large bending radii (40-100 mm), dramatically increase crosstalk, even if the cores have different propagation parameters (T. Hayashi et al, Crosstalk variation of multicore fibre due to fibre bend, Proc. ECOC2010, 19-23 September, 2010, Torino, Italy, paper We.8.F.6.). Thus, the above structures of multicore fibers apparently either cannot be used in real communication lines, or will require special sophistication and cost increase of the structure of fiber-optic cables, needed to restrict the possible bending of the fiber in the laid cable.
The present invention is aimed at eliminating the above disadvantages of the prior art. The object of the present invention is to provide a multicore fiber comprising a plurality of light-guiding cores having the same or different parameters (diameters or refractive indices) and a barrier layer, whose refractive index must be less than the refractive index of each of the inner reflecting claddings surrounding the respective light-guiding core; so the interaction between modes of adjacent light-guiding cores can be reduced, thereby significantly reducing the optical crosstalk and enabling the reduction in distance between the cores. Reducing the distance between the cores will allow increasing the number of cores in a multicore fiber with the same external diameter of the fiber; therefore the existing underground utilities can be utilized to transmit greater amounts of data.
Furthermore, the presence of the barrier layer will reduce the bending optical loss in each of the cores and thus improve the quality of information transmission.